Steam methane reforming processes are widely used in the industry to make hydrogen and/or carbon monoxide. Typically, in a steam reforming process a fossil-fuel hydrocarbon containing feed such as natural gas, steam and an optional recycle stream such as carbon dioxide, are fed into catalyst-filled tubes where they undergo a sequence of net endothermic reactions. The catalyst-filled tubes are located in the radiant section of the steam methane reformer. Since the reforming reaction is endothermic, heat is supplied to the tubes to support the reactions by burners firing into this radiant section of the steam methane reformer. Fuel for the burners mainly comes from by-product sources such as purge gas from pressure swing adsorption (PSA), and some make-up natural gas. The following reactions take place inside the catalyst packed tubes:CH4+H2O⇔CO+3H2 CH4+CO2⇔2CO+2H2 CO+H2O⇔CO2+H2 
The crude synthesis gas product (i.e., syngas) from the reformer, which contains mainly hydrogen, carbon monoxide, and water, is further processed in downstream unit operations. An example of steam methane reformer operation is disclosed in Drnevich et al (U.S. Pat. No. 7,037,485), and incorporated by reference in its entirety.
Syngas exiting the steam methane reformer is at high temperature, typically between 1450-1650° F., depending on the plant rate and product slate. Outside the heated zone of the reformer, syngas from the individual tubes is collected and sent downstream for further processing in the aforementioned unit operations. In reformers where the tube outlets are not encased in refractory or placed in refractory lined enclosures, the exposed flanged tube outlet is typically fitted with both internal and external insulation. The design of the tube outlet assembly insulation is critical to preventing premature tube failure as insufficient insulation can lead to temperatures favorable for metal dusting in some areas of the tube outlet, and dew point condensation-related failures in other sections. On the other hand, too much insulation can result in high temperatures at the flanges and eventual weakening or decarburization. The external insulation comprises a high temperature fibrous insulation blanket wrapped around the tube outlet. The internal insulation is sheet metal formed into a shape, hereinafter referred to as a can, and filled with high temperature fibrous insulation material. One end of the can is securely attached to a blind flange such as by welding, and the other end is sealed to enclose the insulation material. The can is positioned inside the reformer tube with a clearance or gap, which as utilized herein refers to the spacing between the outside surface of the can and the inner wall of the reformer tube.
Garland et al (U.S. Pat. No. 8,776,344 B2) disclose a cylindrical can with an angled base, and a ‘seal’ for use in the inlet of a reformer tube assembly. In a reforming furnace, hot feed gas (typically <1300° F.) is delivered into the individual reformer tubes. In tube assemblies where the inlet port enters from the side, it has been discovered that the hot process gas swirls on entering the tube and some gas can flow upwards toward the flanges, causing them to overheat. This is detrimental to the lifespan and performance of the reformer tubes. The cylindrical, angled base plug disclosed in this patent is positioned adjacent to the inlet port to direct the fluid introduced through said inlet port away from the flanges. The seal placed in the gap limits passage of hot fluid upwards along the gap, thereby preventing overheating of the flanges. However, the invention of the Garland et al disclosure is only applicable to the reformer tube inlet assembly. It aims to reduce flange and weld neck temperatures of the tube inlet. No considerations were given to metal dusting or hydrogen attack of the tube inlets as there is no carbon monoxide (CO) and very little hydrogen (H2) in the process feed gas.
While Hohmann et al (U.S. Pat. No. 5,490,974), Roll et al (U.S. Pat. No. 5,935,517) and Boll et al (U.S. Pat. No. 6,099,922) disclose some methods for preventing metal dust corrosion in outlet pipes and headers containing syngas, the disclosures in these documents concern only outlet pipes and headers that are lined with refractory on the inside. In such cases, carbon monoxide can diffuse through the refractory and come into contact with sections of the metal whose temperatures are in the metal dusting favorable range. This can lead to carburization and catastrophic failure of the material. In the '974 and '517 documents, a hot gas purge is applied to the refractory to arrest syngas diffusion and prevent metal dusting. In the '922 document, the refractory is infused with nickel-based catalyst that promotes reaction of carbon monoxide with the hydrogen and water in the syngas to form CO2, H2O, H2 or CH4, thereby eliminating the potential for metal dust corrosion.
For reformer furnaces in which the tube outlets are exposed to the ambient, the insulation design is critical to preventing a deleterious temperature profile. In the presence of high CO partial pressures, as typically would occur in a reformer tube, areas of the tube inner wall metal surfaces at temperatures between 900-1400° F. are susceptible to high rates of metal dusting. Also, it is important that the wall temperatures stay above the dew point temperature of the syngas to prevent dew point condensation related failures. However, putting too much insulation on the tube outlet to avert the two aforementioned material failure mechanisms will result in high flange temperatures which can lead to decarburization or weakening and cracking of the steel. Premature tube failure can result in extended, unplanned plant shutdown and possible contractual penalties.
Thus, to overcome the disadvantages in the related art, one of the objectives of the present invention is to provide an internal insulation design to the tube outlet assembly that leads to a desired tube metal temperature profile.
It is an object of the invention that the tube outlet assembly insulation ensures that areas of the tube outlet with temperatures favorable to metal dusting occur only in low syngas flow areas in the annular gap between the internal insulation can and reformer tube inner wall in order to greatly minimize the rate of metal dusting corrosion.
It is another object of the invention that the tube outlet assembly insulation reduces the convection of hot syngas to the flanges thereby reducing flange temperatures and preventing high temperature hydrogen attack of the steel flanges.
It is a further object of the invention to prevent dew point condensation related failures by maintaining the entire length of the tube outlet above the syngas dew point temperature.
Other objects and aspects of the present invention will become apparent to one skilled in the art upon review of the specification, drawings and claims appended hereto.